As a response to allergen exposure in allergic conditions, mast cells are activated and release mediators like histamine, thromboxane A2 (TxA2), cysteinyl leukotrienes (CysLTs) and prostaglandin D2 (PGD2). These mediators interact with their respective receptors and cause physiological effects such as increased vascular permeability, edema, pruritus, nasal and pulmonary congestion, bronchoconstriction, and mucus secretion. An increased vascular permeability for example, allows excessive infiltration of eosinophilic and basophilic leukocytes into the tissue and thus amplifies the allergic response. Current treatments of allergic diseases comprise agents that can block or otherwise interrupt such interactions, e.g. anti-histamines (histamine H1 receptor antagonists), leukotriene receptor antagonists, beta-adrenergic receptor agonists, and corticosteroids. Generally, treatments with anti-histamines and leukotriene antagonists are limited in efficacy, and long-term usage of corticosteroids is often associated with unwanted side effects.
PGD2 is an agonist known to act on two G-protein-coupled receptors, the PGD2 receptor DP1 and the recently identified CRTH2 (chemoattractant receptor-homologous molecule expressed on Th2 cells) receptor (also referred to as “DP2 receptor”).
Elevated PGD2 levels are considered to cause inflammation as observed in allergic diseases such as allergic rhinitis, allergic asthma, allergic conjunctivitis, atopic dermatitis and the like. Therefore, blocking the interaction of PGD2 with its receptors is considered a useful therapeutic strategy for the treatment of such diseases.
GB 2388540 discloses the use of ramatroban ((3R)-3-(4-fluorobenzene-sulfonamido)-1,2,3,4-tetrahydrocarbazole-9-propionic acid), a TxA2 receptor (also referred to as “TP receptor”) antagonist with additional antagonistic activity on CRTH2, for the prophylaxis and treatment of allergic diseases, such as asthma, allergic rhinitis or allergic conjunctivitis. In T. Ishizuka et al., Cardiovascular Drug Rev. 2004, 22(2), 71-90 effects of ramatroban on late-phase inflammation are described. Furthermore, oral bioavailability of ramatroban and its ability to inhibit prostaglandin D2-induced eosinophil migration in vitro has been reported (Journal of Pharmacology and Experimental Therapeutics, 305(1), p. 347-352 (2003)).
WO 03/097598 and WO 03/097042 disclose Ramatroban analogues with CRTH2 antagonistic activity. Ulven et al, J. Med. Chem. 2005, 48(4), 897-900 disclose further ramatroban analogues.
CRTH2 antagonists containing a phenoxy-acetic acid moiety have been for instance described in WO 04/089885, WO 05/105727, WO 06/056752, WO 07/037187 and WO 07/052023. 1-Phenyl-1,2,3,4-tetrahydroisoquinoline derivatives have been disclosed in WO 2012/004722.